1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring positions of a pattern formed on a substrate such as a photomask or a reticle.
2. Related Background Art
According to a conventional pattern position measuring apparatus, a measured substrate having its surface formed with a pattern is supported on support parts disposed in four positions on a stage. The measured substrate is adsorptively supported with its pattern forming surface being directed upward. Pattern edges are detected while moving the stage two-dimensionally. At the same time, two-dimensional positions of the stage are read from interferometers, thus measuring the pattern positions.
The measured substrate supported on the stage is, however, flexed by its tare weight. When supported on the stage with its pattern forming surface being directed upward, the pattern forming surface is shrunk. The conventional apparatus is therefore incapable of measuring only the pattern positions in a shrunk state of the pattern forming surface.
Under such circumstances, according to a pattern position measuring apparatus disclosed in U.S. Pat. No. 4,730,927, slopes of the surface of the substrate in the measured pattern positions are obtained. Computed are two-dimensional displacement quantities of the pattern positions that are caused due to these slopes. The measured pattern positions are corrected to pattern positions in a flat state.
Given herein is a brief explanation of a flexure correcting method disclosed in U.S. Pat. No. 4,730,927.
FIG. 12 is a view illustrating a supported state of the substrate in the conventional pattern position measuring apparatus. Support parts 3a, 3b, 3c, 3d are provided in four positions on a stage 2. A substrate 90 is drawn to the support parts 3a, 3b, 3c, 3d. FIG. 13 is an enlarged sectional view taken substantially along the arrowed lines 13--13' in FIG. 12. The substrate 90 is, as illustrated in FIG. 13, flexed downward by its tare weight. The pattern forming surface is slightly shrunk.
A correction of the two-dimensional displacement quantities of the pattern positions involves the step of at first measuring heights of a pattern position 92a on the surface of the substrate 90 and of substrate surface positions 93, 94 in the vicinity thereof from a reference plane (e.g., a moving horizontal plane of the stage). An X-directional slope .theta. in the pattern position 92a is given in approximation such as: EQU .theta.=tan.sup.-1 [{(W.sub.1 -W.sub.0)/l+(W.sub.2 -W.sub.0)/l}/2]
where W.sub.0 is the height of the pattern position 92a from the reference plane, W.sub.1, W.sub.2 are the heights of the substrate surface positions 93, 94 in the vicinity thereof from the reference plane, and l is the spacing between the respective positions in the horizontal direction. The slope of the substrate surface in the pattern position 92a is equal to a slope of a neutral surface 91 in a vertical downward position 92b aligned with the pattern position 92a. Hence, an X-directional displacement quantity of the pattern position 92a due to the slope .theta. of the surface of the substrate 90 can be obtained by: EQU (1/2).times.t.times..theta.
where t is the thickness of the substrate 90.
Similarly, a Y-directional displacement thereof is obtained.
The measured position of the pattern 92a in the shrunk state of the pattern forming surface is corrected to a pattern position in the ideal flat state in accordance with the thus computed X- and Y-directional displacement quantities.
There arises the following problem inherent in the prior art described above. Detected are only the pattern position in the shrunk state of the surface (pattern forming surface) of the measured substrate and the pattern position in the ideal flat state with no flexure and no deformation. For instance, in a semiconductor aligner using a reticle, the reticle is supported with its pattern forming surface set downward. It therefore follows that the pattern forming surface elongates reversely when supported on the pattern position measuring apparatus. The conventional apparatus is incapable of detecting the pattern positions when supported on the semiconductor aligner.
Another problem peculiar to the prior art described above is a decline of throughput. It is because the heights of the pattern measuring position and the positions vicinal thereto have to be respectively measured.
It is a first object of the present invention, which has been devised in view of the foregoing problems inherent in the above-mentioned prior art, to provide a pattern position measuring apparatus capable of correcting the measured pattern position to pattern positions in a variety of flexural states in different support modes.
It is a second object of the present invention to provide a pattern position measuring apparatus capable of exhibiting a high throughput.